1. Field of the Invention
The invention relates to a hierarchic synchronization method for exchanges in a mesh telecommunication network.
2. Description of the Related Art
Mobile automatic telephone exchanges are frequently used for military purposes and also be certain customers such as the police, civil defense, large building sites. In such a mobile automatic telephone system all the subscribers can be reached directly and without delay by automatic dialling in their own area and also in similar, neighbouring areas. In such a mobile automatic exchange system it is a requirement that the network can be established and changed rapidly, so that for the connection set-up the required exchange units can be selected in accordance with the instantaneous operating conditions and a change in location of the subscriber is possible without a disturbance in the network organization.
Each subscriber has its own dialling number, which depends on where in the network the subscriber is present at that moment. The mobile automatic exchange system must then be adapted to the variable expansion of the network, to the changing network structure, to the significantly fluctuating traffic load and to the changing availability of connecting paths.
A multi communication system MKS 200 is extensively described and explained in the Philips publication PKI Technische Mitteilungen No. 1/1985, pages 18 to 26. The configuration of the network using a homing method and the requirements made on the network synchronization are described in detail in chapter 3 "Netzaspekte", page 23, centre column, to page 26, left-hand column. As regards the network synchronization, it is stated on page 25, left-hand column that the configuration of the clock distribution network (clock tree) is effected in the central character channel by means of network management reports.
A number of different synchronization methods are known for the network synchronization, which are described in greater detail in the publication Fernmelde-Praxis, Band 59 dated Nov. 25, 1982, pages 887 to 910, more specifically page 892. As regards the hierarchic master-slave method it is described on page 892 that this method is particularly suitable for mesh networks. An exchange then functions as the master with its own independent clock, which is used to control the clocks of the sub-ordinate slave-exchanges. In this situation, an identification is assigned to each clock in accordance with its position in the hierarchy, the slave clock which is next in the hierarchy being automatically used for the synchronization when the master clock fails.
In the hierarchic master-slave synchronization method, information about the hierarchic status of the exchanges must continuously be transmitted and evaluated in each exchange. In a mobile automatic exchange system comprising a plurality of exchanges, this results in a heavy traffic load in the exchanges during the transmission and evaluation of the synchronization reports transmitted in the central character channels. When the master clock fails or the connecting line between the master exchange and the slave exchange fails, the re-establishment of the synchronization requires in the event of a long identification, a great deal of time to re-detect the master clock.
The Fernmelde-Praxis Publication does not contain any further explanation of how and in what manner the slave clock next highest in the hierarchy is obtained on failure of the master clock.